1. Field of the Invention
The present invention generally relates to distance measurement, and more specifically to a system and method for distance measurement by inphase and quadrature signals in a radio system.
2. Related Art
Distance measurement systems are required in a wide variety of applications. Surveying, position determination, precision farming and mining are some example applications that require distance measurement systems.
In recent years, various systems for distance measurement systems have been developed. The global positioning satellite (GPS) system operated by the United States Department of Defense, for example, is a highly complex system of determining the position of an object by distance measurement. The GPS system depends on measuring the time-of-flight of signals from three or more orbiting satellite transmitters by a navigation receiver. According to the GPS system, each satellite broadcasts a time-stamped signal that includes the satellite""s ephemeris, i.e., its own position. When the mobile unit receives a GPS signal, the mobile unit measures the transmission delay relative to its own clock and determines the distance from the transmitting satellite""s position.
Another approach is that employed by the U.S. Navy""s TRANSIT system. In that system, a mobile unit performs continuous doppler measurements of a signal broadcast by a low earth orbit (LEO) satellite. The measurements continue for several minutes. The system usually requires two passes of the satellite, necessitating a wait of more than 100 minutes. In addition, because the distance and position calculations are performed by the mobile unit, the satellite must broadcast information regarding its position, i.e., its ephemeris. Although the TRANSIT system is capable of high accuracy (on the order of one meter), the delay required is unacceptable for commercial applications.
Although these systems accurately measure the distance between objects, they are extremely complex, and, more importantly, expensive to implement. For example, both the GPS and TRANSIT systems require multiple satellites, sophisticated receivers and antennas that require hundreds of millions dollars of investments. Also, response times of GPS and TRANSIT systems are typically slow due to their narrow bandwidth. Furthermore, since these systems depend on orbiting satellites, they require an unimpeded view of the sky to effectively perform range measurement. For these reasons, it has been recognized that there is a need for a simple, low cost distance measurement system.
The present invention is directed to a system and a method for distance measurement using a radio system. According to the present invention, a distance is measured by determining the time it takes a pulse train to travel from a first radio transceiver to a second radio transceiver and then from the second radio transceiver back to the first radio transceiver.
The actual measurement is a two step process. In the first step, the distance is measured in coarse resolution, and in the second step, the distance is measured in fine resolution.
In accordance with the present invention, a first pulse train is transmitted using a transmit time base from the first radio transceiver. The first pulse train is received at a second radio transceiver. The second radio transceiver synchronizes its time base with the first pulse train before transmitting a second pulse train back to the first radio transceiver. The second pulse train is received at the first radio transceiver, which then synchronizes a receive time base with the second pulse train.
The time delay between the transmit time base and the receive time base can then be determined. The time delay indicates the total time of flight of the first and second pulse trains. The time delay comprises coarse and fine distance attributes.
The coarse distance between the first and second radio transceivers is determined. The coarse distance represents the distance between the first and second radio transceivers in coarse resolution. An inphase (I) signal and a quadrature (Q) signal are produced from the time delay to determine the fine distance attribute. The fine distance indicates the distance between the first and second transceivers in fine resolution. The distance between the first and second radio transceivers is then determined from the coarse distance and the fine distance attributes.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.